Furnace, its method of operating and control

ABSTRACT

The present invention relates to a furnace ( 10 ), its method of operation and control. The invention overcomes problems associated with existing furnaces by improving the recovery rate of waste metal. In a preferred embodiment the furnace ( 10 ) comprises a cylindrical body of constant internal diameter. The furnace body ( 12 ) is mounted on a frame ( 15 ) pivoted to a ground members ( 16   a  and  16   b ), the furnace body ( 12 ) is adapted to be reclined or inclined or at various angles (α and ⊖); a burner ( 30 ) to heat the furnace, and a door ( 19   a,    19   b ) for sealing an open end ( 14 ). As the internal walls of the furnace body ( 12 ) are of a constant diameter, it is no longer necessary to incline the furnace ( 10 ) to such a degree in order to pour molten metal, because there is no narrow neck (which previously acted like a weir). In a preferred embodiment combustion air is routed through the door hinge to the burner ( 30 ). As a result the air/fuel delivery system has gas tight rotary and elbow joints is attached to the furnace ( 10 ) and tilts and moves with the furnace ( 10 ). An artificial intelligence system monitors process variables and controls the operation of the furnace ( 10 ).

FIELD OF THE INVENTION

The present invention relates to a furnace, its method of operation andcontrol.

More particularly the invention relates to a furnace, to a method ofoperating a furnace and to a method of controlling a furnace in order torecover nonferrous metals, such as, for example, and without limitation:copper, lead and aluminium. The invention is particularly well suitedfor the recovery of aluminium.

BACKGROUND

Furnaces for the recovery of metals, such as aluminium, are well known.Increasingly there is a demand for such furnaces, as legislation tendsto encourage recovery and recycling of materials, particularly wastemetals. There are also environmental benefits in recovering wastemetals, rather than mining and smelting virgin ore. Aluminium isparticularly well suited for mixing recovered (waste) aluminium with newaluminium material.

For the purposes of the present specification and the understanding ofthe invention, the furnace, its methods of operation and control will bedescribed with reference to recovery of aluminium. However, it will beunderstood that variation to materials, operating conditions andparameters may be made so as to modify the furnace in order to enablerecovery of other non-ferrous metals.

Furnaces for recovering waste aluminium have a heating system whichmelts the aluminium. A flux is introduced into the furnace to assistwith the aluminium recovery. The flux generally consists of NaCl andKCl, other chemicals such as cryolite, may be added to the flux. Theflux or salt cake assists in the process and is a well-known art. Atelevated temperatures, typically from 200° C. -1000° C., the melted fluxfloats on the molten aluminium, as it is less dense. Pouring ofrecovered liquid aluminium is then possible by tipping or tilting thefurnace in such a way that the flux remains in the furnace.

PRIOR ART

Existing metal recovery furnaces have a generally cylindrical body whichis pivoted to a stand so that it can move from a first, predetermined,generally horizontal heating phase position (whilst aluminium ismelting) to a second, inclined pouring position, at which positionmolten aluminium can be poured. Some existing furnaces have bodies thathave an open end that tapers inwards. Waste aluminium is loaded into thefurnace and molten aluminium is poured from the furnace at the open end.

An example of a metal recovery furnace with an inwardly tapered open endis described in European Patent Application EP-A3-1243663 (Linde AG). Aprocess for melting contaminated aluminium scrap is described. Theprocess comprises: measuring the oxygen content of waste gas produced onmelting the scrap; and using the value as a control parameter duringpyrolysis of the impurities and/or during melting of the aluminium.

Other types of furnace were fitted with one or more furnace doors. Thefurnace door(s) were provided at the open (pouring) end of the furnace.Sometimes furnace doors supported a furnace heater. The door(s) was/werehinged to a fixed point separate from the cylindrical body of thefurnace. Therefore it was only possible to close the furnace doors whenthe cylindrical body of the furnace was in a predetermined position.

A requirement was that the furnace was able to adopt a predeterminedposition in order to retain molten metal. The fact that existingfurnaces had to adopt this position meant that the furnace could only beoperated at one angle. This was to some extent alleviated by using aninwardly tapered open end, which defined a reservoir within the furnacein which melted aluminium flowed. When it was desired to pour out themelted aluminium, for example into a launder (refractory receptacle),sometimes the flux poured out with the molten material because it wasdifficult to separate the flux from the molten aluminium. One reason forthis was that existing furnaces had to be tipped to such an angle inorder to cause or permit molten aluminium to be poured. The result wasthat a mix of flux and molten aluminium were sometimes poured and ascraper was often required to separate the two. Also, to some extent thetapered end reduced the size of the open end of the furnace body,thereby limiting the size of objects, which could be placed in thefurnace.

With the door closed it was not possible to view the melting process.Inadvertent opening of the door lead to an exothermic reaction,resulting in the aluminium being burnt away upon reaction with excessoxygen.

The invention provides a furnace that overcomes the above problemsassociated with existing furnaces.

Another object of the invention is to provide a furnace which has agreater recovery rate of waste metal than has hereto for beenachievable.

SUMMARY OF THE INVENTION

According to the present invention there is provided a furnacecomprising: a generally cylindrical furnace body having a closed andopen end of generally constant diameter, a frame pivoted to a groundmember, said frame supporting the furnace body for rotation at variousangles in a reclined position away from the open end and in an inclinedposition towards the open end, a burner to heat the furnace, and a doorto seal the open end.

As a result of the generally constant diameter of the internal walls ofthe cylinder of the furnace it is no longer necessary to incline thefurnace to such an exaggerated angle in order to pour molten metal. Inaddition, once poured a much higher percentage of molten metal can beobtained, because there is no longer confinement of residue within thefurnace as a result of a lip or neck.

Ideally the door is hinged to the frame that supports the furnace and iscapable of displacement in unison with the inclining (raising andlowering) of the furnace. An advantage of this is that the doors arealways maintained in close proximity with the mouth of the furnace. Thebeneficial effects of this are two fold: firstly there is less risk ofoxygen entering the furnace (which could contaminate the atmosphere) andsecondly, because the furnace is maintained in a closed state during itsoperation, heat losses are reduced. Thus efficiency is increased, asless energy is required to melt the aluminium. Therefore it is apparentthat the use of the invention provides a cost effective (and moreprofitable) aluminium recovery process.

Preferably the, or each, door has one or more inspection hatches to viewthe melting process and/or through which molten material can be poured.Because the area of the, or each, inspection hatch(es) is (are) smallerthan the door itself, less energy escapes on inspection of the inside ofthe furnace.

Advantageously the, or each, door has two halves hinged to either sideof the frame. In an exemplary embodiment the hinges act as integralair/fuel delivery ducts enabling the furnace doors to be closed andheating to take place in a controlled atmosphere.

Preferably the heater is a gas burner and is mounted on the door ashereinafter described. In a particularly preferred embodiment thecombustion air is routed through the furnace door hinge to the burner.The air and fuel gas delivery system (air and gas train) is attached tothe furnace and is also able to tilt and move with the furnace. This isachieved using elbow and/or rotary fluid connections employing rotaryjoints that are gas tight.

According to another aspect of the invention there is provided a furnacecomprising: a generally cylindrical furnace body having a closed andopen end of generally constant diameter; a frame pivoted to a groundmember, said frame supporting the furnace body for rotation at variousangles in a reclined position away from the open end and in an inclinedposition towards the open end, there being a door which opens and closesby swivelling on a hinge and a burner for heating the furnace, wherebyair and/or gas is delivered to the burner by way of a manifoldsupported, by or passing through, the hinges.

This is achieved using elbow and/or rotary fluid connections employingrotary joints that are gas tight. As a result the air and fuel gasdelivery system (air and gas train) is able to tilt and move with thefurnace.

The burner is ideally mounted in one door, at an angle and in such a waythat a gas jet, emanating therefrom, does not impinge on the payloadmaterial being processed. An advantage of this is that heat is neverapplied directly to the payload. Therefore, unlike with existingfurnaces, there is less risk of oxidising the molten metal to berecovered. The corollary of this is that yield is further improved.

Conveniently the burner is a high velocity type burner, but other typesof burners may be employed. Typically the thermal rating of the burneris determined by the size and throughput of the furnace, but is notusually less than 1200 kW.

The angle of the burner mounted in the door or doors is such that itensures optimum heat transfer into the refractory and into the materialbeing processed and ideally aims the jet towards the end wall of theinterior of the furnace body.

Preferably the furnace has an exhaust port. An air jet or air curtain isprovided across the exhaust port to control the pressure within thefurnace. The air jet or air curtain enables pressure balancing of theinternal atmosphere of the furnace with respect to the externalatmosphere. This feature further enhances energy efficiency and recoveryas the air curtain effectively seals the furnace, thereby reducingoxygen in the internal atmosphere, thus reducing oxidation. Moreoverbecause there is a sealing effect, less energy is lost from the furnace,for example as a result of convection losses. Thus the air curtain atthe furnace door exhaust helps to control the furnace pressure andfurnace conditions. The air curtain is preferably dimensioned andarranged to suit the size of furnace and application.

Artificial intelligence control system, such as a fuzzy logic neuralnetwork control system, controls important process variables and processsub-variables are described below.

Conveniently one or more sensors is/are provided to sense thetemperature of a refractory liner and molten material.

Temperature sensors in the furnace doors are directed at refractorylinings and/or material being processed to measure the temperature ofthe refractory and material being processed. Knowledge of the externalfurnace skin temperature and distribution of heat across the exteriorsurface of the furnace, enables greater control of the heating regime.

A plurality of sensors, placed in a known relationship one with another,enable averaging of furnace temperature to be obtained as well asproviding important information as to thermal transients in the furnacetemperature.

Conveniently a circumferential ring supports a toothed gear which isconnected to a drive system. The drive system may comprise a drive motoror is chain driven and is adapted to engage with sprockets or gear teethdisposed around an outside surface of the furnace. Where a chain driveis used ideally the number of sprocket teeth on the circumferentialring, around the furnace girth, is half that of the chain pitch. Thisreduces drag and chain wear and therefore reduces power requirement ofthe drive motor. Additionally the lives of the chain and sprocket areincreased.

Packaging wedges are ideally employed to ensure a close fit between acircumferential ring (on which the furnace rotates), and the outersurface of the furnace. These wedges are ideally connected using athreaded member which when tightened causes the wedge to pinch the ringand ensure tight grip concentric with surface mounted lugs and the ring.This is necessary due to differential thermal expansion that occurs whencycling the furnace through its operating regime.

Ideally the drive motor can rotate the furnace at a variable rotationalspeed. The rotation of the furnace serves to churn the material beingprocessed and transfer heat into the material via the refractory.Ideally, agitation is achieved by rotating and counter rotating thefurnace, (this is achieved by rapid actuation of an alternating current(AC) electric motor), at predetermined and selected operating angles andspeeds.

The electric motor is connected to the furnace as mentioned aboveeither: by way of a fixed linkage such as a gear, rack and pinion; orideally a chain drive. The combination of electric motor, motorcontroller and linkage mechanism is hereinafter referred to as a furnacerotation system. The furnace rotation system is advantageouslycontrolled for braking purposes by using a dynamic braking system. Aninverter is used to control the motor for braking purposes and directcurrent (DC) is controllably injected as part of a dynamic brakingsystem.

The dynamic braking system involves the steps of: injecting directcurrent (DC), under control of a feedback loop, based upon a signalwhich is obtained from one or more sensors sensing load characteristicsof the furnace. Such furnace load characteristics include: requiredtorque and smoothness of rotation. In order to rapidly decelerate thefurnace, a controller obtains a DC value based upon the configuration ofthe invertors, parameters and outputs a feedback signal which is used tocontrol the level and rate of injection of the DC for slowing the motorand/or holding the motor in a particular orientation. The furnace andits contents are thereby held in a predetermined position. As the moltenmetal is denser than the flux the metal drops to a lower region of thefurnace from where it can be readily poured or counter rotated toachieve optimum mixing of waste material and flux (churning).

Because the walls of the interior of the furnace are parallel andcylindrical with a furnace door covering the open end of the furnace,pouring of the melt at a lower angle of inclination (tipping angle) isachieved. When this is desired the furnace is inclined preferably byextending two hydraulic rams or jacks.

According to a yet further aspect of the invention there is provided amethod of operating a furnace comprising the steps of: loading thefurnace with a mixture of flux and a material to be melted, from whichmetal is to be recovered; heating the mixture until the metal melts;agitating the mixture so as to promote agglomeration of the moltenmetal; and inclining one end of the furnace in order to pour the moltenmetal.

The method of operating the furnace may be repeated by reclining theraised end, introducing fresh material to be melted, from which metal isto be recovered, agitating the mixture so as to promote agglomerationand raising one end of the furnace in order to pour recovered metal.

Preferably the angle of inclination is less than 20°, more preferablythe angle of inclination is less than 15°, most preferably the angle ofinclination is less than 10°.

According to a yet further invention there is provided a method ofcontrolling a furnace comprising the steps of: controllably heating afurnace, by controlling at least the following conditions: thetemperature; the mass of payload; the viscosity of the payload; the timeto reach the viscosity; the atmospheric oxygen content of the furnace;the rate of application of energy and the cumulative energy applied.

The furnace door, or doors, is/are fitted with inspection doors orhatches that can be opened during the process to check the condition ofthe material being processed with a minimum release of energy. However,monitoring of the aforementioned variables is ideally achieved by way ofa plurality of sensors and a remote data acquisition system such as aSupervisory Control And Data Acquisition, (SCADA) system. Ideally theSCADA system is incorporated in furnace control equipment and collectsand analyses all furnace data and control inputs and outputs.

Use of SCADA systems enables on-line diagnosis of the process and remoteaccess support. This aspect of the invention improves on-line monitoringand electronic archiving. A dedicated field communication data buswiring system for example PROFI-BUS (trade Mark) is ideally used inpreference to multi-core cabling networks. Local and remote controlboxes receive and encode signals for process sensors which are ideallypositioned to measure process variables incorporated into the furnaceprocess control system, for example and without limitation, furnace skintemperatures, refractory temperatures, fuel gas and air flows andpressures.

Preferably the angle of the frame is altered by means of hydraulicram(s) whereby to support the body for rotation at various angles in areclined position away from the open end and in an inclined positiontowards the open end. The hydraulic rams are ideally water-glycol heatresistant type.

Preferably the frame is pivoted to the ground member such that thepivotal axis is in alignment with a pouring lip at the open end of thefurnace body.

Preferably the furnace is adapted to recover waste aluminium.

All of the aforementioned contribute to higher metal recovery yields,lower energy usage, lower flux usage and faster cycle times.

The furnace combustion system can operate on several fuels, natural gas,propane, heavy fuel oil, light fuel oil, oxy fuel etc.

BRIEF DESCRIPTION OF THE FIGURES

An embodiment of the invention will now be described with reference tothe accompanying drawings in which:

FIG. 1 shows a perspective view of a preferred embodiment of a furnace(with the door removed) showing a furnace body, a support frame and adrive system;

FIG. 2 shows a side view of the furnace shown in FIG. 1, with thefurnace at a reclined angle (α);

FIG. 3 shows a side view of the furnace shown in FIG. 1, with thefurnace in a raised position for tipping or pouring, at an inclinedangle (β);

FIG. 4 shows a part section view along line X-X of FIG. 5, showing asection of one of typically 18 packing wedges urged in contact against asteel “tyre” surrounding the furnace;

FIG. 5 is a view along arrow Y of FIG. 4, showing a plan view of one ofthe packing wedges urged in contact against the steel “tyre” surroundingthe furnace;

FIG. 6A shows a front view of the door of the furnace;

FIGS. 6B and 6C show side views of the door of the furnace;

FIG. 6D shows a diagrammatical above plan view of the doors of thefurnace (in both open and closed positions), so as to illustraterotating air and gas inlet manifolds;

FIG. 7 a is a system structure illustrating “fuzzy” logic inference flowprocesses for some examples and (without limitation) key decision stepsin an artificial intelligence system;

FIG. 7 b is a chart illustrating membership functions, for example, ofsome variables, and (without limitation) some key decision steps in anartificial intelligence system; and

FIG. 7 c is a flow diagram illustrating feedback control from theartificial intelligence system to gas and air supplies to the furnaceand shows how furnace temperature is raised/lowered.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the Figures generally and FIGS. 1 to 3 in particular, thereis shown a furnace 10. Furnace 10 has a generally cylindrical furnacebody 12 of generally constant external diameter and internal diameter,as a result of parallel sidewalls. Furnace body 12 has a closed end 13and an open end 14. Body 12 may be formed from steel and linedinternally using refractory linings or bricks as is well known in theart. Examples of refractory linings or bricks are STEIN 60 P (TradeMark) and NETTLE DX (Trade Mark).

The frame 15 is provided to support the furnace body 12 for clockwiseand counter clockwise rotation as shown by arrows A. To rotate body 12,frame 15 may include support wheels on which the body 12 rests and amotor 20 driving a toothed wheel 22 on the body 12. Torque istransmitted from the motor 20 to the toothed wheel by way of a chain 24.

Frame 15 is pivoted to a ground support member in the form of feet 16Aand 16B secured to the ground, providing a pivotal axis “Z-Z”. The frameangle can be altered relative to the feet 16 a, 16 b such that the frame15 can support the body 12 for rotation at various angles (α) from thehorizontal, in a reclined position away from the open end (furnacemouth) and (β) in an inclined position towards the open end. The angleof inclination of the frame is altered by means of hydraulic rams 16 c,16 d. Hydraulic rams 16 c and 16 d are ideally of the water-glycol heatresistant type.

Furnace body 12 has a pouring lip 17 at the lowest point of the open end14, and the pivotal axis “Z-Z” is in alignment with a pouring lip 17 atthe open end 14 of the furnace body 12.

As shown in FIGS. 6 a, 6 b and 6 c, frame 15 has at one end a doorsupport structure 15 a to which is hinged a door 18 to seal the open end14. Door 18 has two doors 19 a and 19 b hinged to opposing sides of thedoor support structure 15A. Doors can swing away from open end 14 toallow the furnace to be loaded or molten metal to be poured out, or thedoors can swing towards the open end 14 to seal it. In practice there isa gap between the doors and the open end 14 when the doors seal the openend.

A burner 30 is provided on door 19 b. Burner 30 can be fed fuel (such asnatural gas) and air through a feed pipe or duct 31, with gas beingsupplied via a gas rotary joint 32 and air being supplied through an airrotary joint 33. Feed pipe 31, gas rotary joint 32 and air rotary joint33 are collectively referred to as fuel delivery system. The reach ofcombustion gasses from the burner 30 can be as great as 4 m or even 6 min longer furnaces. Because the gas delivery system is effectively ableto move in two orthogonal planes, by way of rotary joints 32 and 33, itis possible to swing open the (or each) furnace door(s), as well as tiltthe furnace on hydraulic rams 16 c and 16 d, with the burner(s) 30operating.

Doors 19 a and 19 b each have an inspection hatch 34 a and 34 b to viewthe melting process and/or through which molten material can be poured.This is an advantage over previously known furnaces as explained above.

Temperature sensors (not shown) are provided to sense the temperature ofa refractory liner and molten material. The sensors are fitted to theoutside of the furnace body 12. An aperture is ideally provided in adoor enabling a sensor to “view” inside the furnace 10. An airflowcooling jacket (not shown) is optionally provided to allow temperaturesensors to operate at low ambient temperatures to prevent damage tothem. The airflow cooling jacket also acts as a purge to keep thesensors and other instrumentation free of dust and smoke and sightvision clean.

Air curtains 45 a and 45 b are provided for each door 19 a and 19 b. Theair curtains 45 a and 45 b enable fine balancing of the internalatmospheric pressure.

Pressure differential between the internal furnace atmosphere andexternal (ambient) pressure can therefore be controlled accurately bybalancing the air curtain(s) across the exhaust port 80.

The furnace 10 has an exhaust port 80 in the door (or doors), and an airjet 50 is provided to control the furnace pressure. The percentageoxygen in the furnace 10 atmosphere is ideally 0% and this is controlledas one of the variables by decreasing air mass flow rate to fuel ratio.By maintaining the percentage of oxygen at or around this level, whenthe aluminium becomes plastic, the risk of oxidation is reduced with theresult that yield is improved.

The furnace 10 is ideally adapted to recover waste aluminium and istherefore loaded in use with NaCl and KCl and in some cases smallamounts of other chemicals such as cryolite to assist in the aluminiumrecovery process.

In use the body 12 of the furnace 10 is reclined away from the open endso that the closed end is lower than the open end. In this position thefurnace is said to be reclined or tilted back. The doors 19 a and 19 bcan swing away from open end 14 to allow the furnace body 12 to beloaded. The wide-open end facilitates this process. The doors 19 a and19 b can then swing towards the open end 4 to seal it. The burner 30 isthen operated to melt the metal in the loaded body 12.

Because the body 12 is reclined, molten metal does not pour out of theopen end. The furnace thus obviates the need to have a small tapered endas with previously known furnaces making for easy loading and theability to load large objects, and most importantly easier and morecomplete pouring of the molten metal. Because the doors 19 a and 19 bare hinged to the frame 15, the doors can be closed whatever the angleof inclination (α or β) of the furnace body. Doors 19 a and 19 b canlater swing away from open end 14 to allow molten metal to be pouredout.

In recycling metal such as aluminium, there are a number of differentvariables. These include: types of flux and percentage thereof, heatapplied (both duration and temperature), melt losses, method ofcharging, types and weight of process material, condition of spent fluxand residual oxides, rotational speed and direction of the furnace bodyand angle of inclination. Other variables that may be used in theoperation and control of the furnace include: the mass flow rate ofcompressed air, ambient air temperature, calorific value of fueldelivered and rate of fuel delivery.

The above mentioned, and possibly other variables, for example whenrecovering other metals, are ideally controlled by a furnace managementsystem, which incorporates a processor (such as a micro-processor in apersonal computer), which may also form part of the furnace of thepresent invention.

Shock loading of the drive motor 20 can be monitored using currentfeedback information form the controller (not shown) of the drive motor20. The nature of the current feedback from driving the motor 20 inorder to rotate the furnace 10 with solid ingots, waste and scrap metalpieces tends to be spiky. As soon as the material melts, and the moltenmaterial agglomerates, the rotational characteristics of the furnace 10becomes much smoother and transients in loading on the motor 20 arereduced eventually disappearing at steady state. Data relating to thisinformation can be used with other variables to determine when it isoptimum to pour aluminium.

Previously operating variable settings were determined by experiencedfurnace operators throughout the process cycle, each individual operatorhaving his own preference for each variable setting or range ofsettings. There has therefore been a loss of consistency in variablesettings during the process cycle with a corresponding variation inmetal recovery rates.

Control and monitoring of the variables directly contribute towardsachieving highest possible recovery rates. As with many engineeringsystems it is not always possible to optimise all variables at the sameinstant during the recovery process. For example, too much heat inputwhen the aluminium is in the plastic or melted stage tends to cause thealuminium to oxidise due to its affinity with oxygen. This greatlyreduces recovery yield. The amount of oxygen in the burner 30 is ideallyreduced at certain stages of the process cycle in order to maximiserecovery. However, this is often at the expense of fuel cost. Thevariables therefore require to be monitored carefully and continuouslyduring and throughout the process.

Experienced operators achieve varying recovery rates. By monitoringvariables and with the use of an artificial intelligence system withoptimised ranges of variables the aspect of the invention which ensuresthat the variable settings are optimised at all times removesinconsistencies from operation and improves yields.

The following lists some of the process variables that are monitored torecycle aluminium:

-   1. The type of flux used and percentage of flux mix in relation to    sodium chloride (NaCl) and potassium chloride (KCl). The percentage    of flux used per type of metal product processed, for example    crushed beverage containers may require more flux than say a large    solid engine block. Processing dross generally requires more flux    than say general aluminium scrap.-   2. The temperature of the flux needs to be controlled during the    process, as does the instant at which fresh flux is introduced and    at what percentage. Determination of when flux is spent is ideally    also made.-   3. The amount of heat required to process different types of product    is an important variable. Temperature requirements for different    types of product may be stored, for example on look-up tables and    used to compute the amount of time required for heating different    types of product.-   4. Exhaust gas temperatures for different alloys are monitored to    provide an indication of the extent of a process.-   5. Melt losses, (the amount of aluminium lost during the process)    provides an indication of the yield of recovery of a process. Prior    knowledge of different melt losses per types of alloys processed may    be used to enhance efficiency of recovery.-   6. The effect of temperature on various alloys; the effect of time    and temperature required for different alloys.-   7. Method of charging process material differs according to the    nature of charging dense and light products and effects of the same.    Percentage weights of product charged for best recovery results.-   8. Condition of spent flux and residual oxides as well as the amount    of aluminium contained in the spent flux. The condition of the spent    flux, residual oxides and the amount of aluminium contained therein    is a process variable which is also influenced by other process    variables. Condition monitoring and information feedback into the    controls system is therefore advantageous.-   9. The rotational speed and incline angle of the furnace. The    rotational speed of the furnace accommodates different products.    Rotational direction of the furnace, (clockwise or anti-clockwise),    during the process. Angle of repose during the furnace cycle is    typically between 0° and 20°.

Referring to FIGS. 7 a,b and c, at least some of the above mentionedvariables, together with others listed below, are identified as beingimportant to the recovery rate and yield of aluminium. The variables (inno particular order of importance) are: refractory temperature, cycletime, recovery rate, metal temperature, flux, heat input, rotationalspeed, material type and alloy, method of loading and furnace tiltangle. Each of the aforementioned main variables have relatedsub-variables. For example, the main variable refractory, depends uponthe following sub-variables: refractory temperature, total heat inputand time period of heat input. Furnace skin temperature depends uponrefractory temperature, the relationship of refractory temperature tofurnace skin temperature over time, the variation in refractorytemperature when Pouring metal, the variation in refractory temperaturewhen charging metal and the refractory temperature when melting flux.

In essence, there may be ten or more main variables and severalsub-variables, on which main variables depend that contribute toachieving the highest possible recovery rates. There are many differenttypes of alloys that can be processed, all requiring individualparameters to optimise recovery rates. It is not possible to optimiseeach variable at any one time during the process, for example, too muchheat input when the aluminium is in the plastic or melted stage willcause the aluminium to burn off due to its affinity with oxygen andtherefore greatly reduce recoveries, this has an effect on the processcycle time. The amount of oxygen in the burner must be reduced atcertain stages of the process cycle in order to maximise recovery but atthe expense of fuel cost and cycle time.

The variables therefore require to be optimised when possible during andthroughout the process. Previously, operating variable settings weredetermined by furnace operators throughout the process cycle, eachindividual operator having his own preference for each variable setting.There was therefore a loss of consistency in the variable settingsduring the process cycle. As a result the metal recovery rates varied.

The control aspect of the invention identifies sub-variables within themain variables and predicts (for example using algorithms or look-uptables) the impact of the main variables and the sub-variables on theoverall process. Alternatively, or in addition to a microprocessor,artificial intelligence (for example in the form of a neural network orfuzzy logic rules) is ideally used to monitor and control the operationof the furnace.

An example of a variable which is controlled will now be described, forillustrative purposes only, with particular reference to FIGS. 7 b and 7c. The particular variable is furnace skin temperature. Sensors 100, 102and 104 sense temperature in three independent locations on the surfaceof the furnace body 12.

Information relating to the temperatures at these locations istransmitted to a SCADA 119, either directly or by way of a noiseresistant bus. Data relating to these variables and other variables istransmitted to microprocessor 120. Microprocessor 120, under control ofsuitable software retrieves information from a look-up table 140 or froma store 130 of membership function data. Membership function data isderived from knowledge of a system's characteristics or may be obtainedfrom interpolation, for example from graphical information of the typeshown in FIG. 7 b. This may be carried out digitally. Using fuzzy logicnetworks, of the type shown in FIG. 7 a, microprocessor 120 computes, inthis particular example any variation or trimming of air flow and/or gas(fuel) flow which may be needed to alter the internal temperature of thefurnace 10.

Control signals generated by microprocessor 120 are transmitted to airpump 150 and gas supply 166 via control lines L1 and L2 respectively.Thus in this particular example knowledge of furnace skin temperaturesT1, T2 and T3 can be used in conjunction with control system 200 toincrease internal furnace temperature (and therefore the temperature ofthe contents of the furnace) by introducing more energy via burner 30.

FIG. 7 b shows a graphical representation of a system structure thatidentifies fuzzy logic inference flow from input variables to outputvariables. The process in the input interfaces translates analog inputsignals into “fuzzy” values. The “fuzzy” inference takes place in socalled rule blocks which contain linguistic control rules. These mayvary according to a particular proprietary system. The output of theserule blocks is known as linguistic variables.

At the output stage the “fuzzy” variables are translated into analogvariables which can be used as target variables to which a controlsystem is configured to drive a particular piece of hardware, such aspump 150, motor 20 or valve 165 on gas supply line 166.

Table 1 in conjunction with FIGS. 7 a and 7 b shows how the “fuzzy”system including input interfaces, rule blocks and output interfaces arederived.

Connecting lines in FIG. 7 a symbolize graphically the flow of data.Definition points on the graph (FIG. 7 b) are shown relating toparticular terms in the Table.

FIG. 7 c shows how the furnace is controlled, by way of an example ofonly one variable—burner control—using information and control signalsderived from the fuzzy logic process. It will be appreciated that manyvariables and sub-variables are simultaneously controlled by the system200 and that control of temperature is described by way of example only.

The invention may take a form different to that specifically describedabove. For example modifications will be apparent to those skilled inthe art without departing from the scope of the present invention.

TABLE 1 Term Name Shape/Par. Definition Points (x, y) very_low linear(0, 1) (10, 1) (15, 0)  (50, 0) low linear (0, 0) (10, 0) (15, 1)  (25,0) (50, 0) medium linear (0, 0) (15, 0) (25, 1)  (35, 0) (50, 0) highlinear (0, 0) (25, 0) (35, 1)  (40, 0) (50, 0) very_high linear (0, 0)(35, 0) (40, 1)  (50, 1)

1. A furnace comprising: a generally cylindrical furnace body having aclosed end and an open end; a frame pivoted to a ground member, saidframe supporting said furnace body for rotation at various angles in areclined position from said open end and in an inclined angle towardssaid open end; a burner to heat said furnace; and, at least one hingeddoor, arranged to close said open end of said furnace; whereinnon-tapered walls of an interior of said furnace are substantiallyparallel and cylindrical from said open end to said closed end; andwherein said at least one hinged door is hinged to said frame and iscapable of inclining and reclining in unison with raising and loweringof said furnace.
 2. A furnace according to claim 1, further comprising:means for raising and lowering said furnace so said furnace body isreclined in a position away from said open end and inclined in aposition towards said open end of said furnace., respectively.
 3. Afurnace according to claim 2, wherein: said means for raising andlowering said furnace comprises a hydraulic ram.
 4. A furnace accordingto claim 1, wherein: said inclined angle is less than 20°.
 5. A furnaceaccording to claim 4, wherein: said inclined angle is less than 15°. 6.A furnace according to claim 4, wherein: said inclined angle is lessthan 10°.
 7. A furnace according to claim 1, wherein: said at least onehinged door has at least one inspection hatch through which moltenmaterial can be poured.
 8. A furnace according to claim 1, furthercomprising: a fuel delivery system attached to said furnace, said fueldelivery system being adapted to raise and lower with said furnace.
 9. Afurnace according to claim 1, further comprising: air and fuel deliveryducts through which combustion air and fuel pass to said burner, saidair and fuel delivery ducts being defined by, or supported in, hinges ofsaid at least one hinged door.
 10. A furnace according to claim 9,wherein: said air and fuel delivery ducts are in fluid communicationwith a fuel delivery system, said fuel delivery system having elbowand/or rotary fluid connections employing rotary joints that are gastight.
 11. A furnace according to claim 1, wherein: said burner ismounted on said at least one hinged door so that, in use, heat isdirected into said furnace body.
 12. A furnace according to claim 11,wherein: said burner is angled with respect to an axis of rotation ofsaid furnace, so that, in use, flame from said burner does not impingeon said payload material being processed.
 13. A furnace according toclaim 1, further comprising: one or more temperature sensors to sense atemperature of a refractory liner and molten material.
 14. A furnaceaccording to claim 1, further comprising: means for generating an aircurtain at said open end of said furnace, which air curtain, in use,permits variation of said internal furnace atmosphere with respect tosaid external (ambient) atmosphere.
 15. A furnace according to claim 1,further comprising: an exhaust port, and an air jet provided across saidexhaust port to control pressure within said furnace, thereby enablingpressure balancing of said internal atmosphere.
 16. A furnace accordingto claim 1, further comprising: a drive motor arranged to rotate saidfurnace at a variable rotational speed.
 17. A furnace according to claim16, wherein said drive motor forms part of a furnace drive systemcomprising: an electric motor; a motor controller; and a linkagemechanism for transmitting torque from said electric motor to saidfurnace body.
 18. A furnace according to claim 17, wherein: saidelectric motor drives said furnace by way of a fixed linkage, said fixedlinkage comprising at least one of a gear train, rack and pinion, and achain drive.
 19. A furnace according to claim 16, wherein: said furnacedrive system acts as a dynamic braking system by way of a controller, aninverter and said drive motor.
 20. A furnace according to claim 17,further comprising: a circumferential ring supporting gear teethconnected to said electric motor with a chain, said chain being adaptedto engage with sprockets or gear teeth.
 21. A furnace according to claim20, wherein: said number of gear teeth is half that of said chain pitch.22. A furnace according to claim 21, further comprising: variablepackaging wedges to ensure a close fit between said circumferential ringand an outer surface of the said furnace body.
 23. A furnace accordingto claim 22, wherein: said packaging wedges are connected using athreaded member which, when tightened, causes said wedge to pinch saidring and ensure tight grip concentric with surface mounted lugs and saidring.
 24. A furnace according to claim 1, further comprising:temperature sensors disposed to measure and to provide an output signalindicative of a temperature of said at least one furnace door, atemperature of refractory linings, and a temperature of material beingprocessed.
 25. A furnace according to claim 1, further comprising: meansfor receiving, encoding and transmitting signals relating to at leastone of the following process variables: furnace skin temperatures,refractory temperatures, fuel gas and air flows, percentage oxygen offurnace atmosphere and internal furnace pressure.
 26. A method ofoperating a furnace, comprising: loading said furnace with a payloadmixture of flux and a material to be melted from which metal is to berecovered; maintaining a controlled furnace atmosphere, by sealing saidfurnace with one or more furnace doors; heating said payload mixtureuntil said metal melts; agitating said mixture so as to promoteagglomeration of said metal by rotating and counter-rotating saidfurnace and by reclining and inclining said furnace; rotating saidfurnace in order to separate flux and molten; and raising one end ofsaid furnace body in order to pour recovered metal.
 27. A method ofoperating a furnace according to claim 26, further comprising: rotatingsaid furnace at a variable speed and inclining said furnace at varyingangles to churn said material so as to assist in a transfer of heat intosaid material.
 28. A method of operating a furnace according to claim26, further comprising: heating said furnace, in accordance with acontrol signal obtained from at least one sensor sensing at least thefollowing: payload temperature; mass of payload; viscosity of saidpayload; time the payload takes to reach viscosity; atmospheric oxygencontent of said furnace; rate of application of energy and cumulativeenergy applied.
 29. A method of operating a furnace according to claim28, wherein: artificial intelligence is used to monitor and controloperation of said furnace.
 30. A method of operating a furnace accordingto claim 29, wherein: a neural network is used to monitor and controloperation of said furnace.
 31. A method of operating a furnace accordingto claim 30, wherein: fuzzy logic rules are used to monitor and controlthe operation of said furnace.
 32. A method of operating a furnace.according to claim 28, further comprising: on-line diagnosis of saidprocess, remote access support, on-line monitoring and archiving.
 33. Amethod of operating a furnace according to claim 32 , wherein: remoteaccess, data acquisition and on-line monitoring is achieved with a SCADAsystem.
 34. A method of operating a furnace, comprising: heating saidfurnace, in accordance with a control signal obtained from at least onesensor sensing at least the following: payload temperature; mass ofpayload; viscosity of said payload; time the payload takes to reachviscosity; atmospheric oxygen content of said furnace; rate ofapplication of energy and cumulative energy applied; loading saidfurnace with a payload mixture of flux and a material to be melted fromwhich metal is to be recovered; maintaining a controlled furnaceatmosphere, by sealing said furnace with one or more furnace doors;heating said payload mixture until said metal melts; agitating saidmixture so as to promote agglomeration of said metal by rotating andcounter-rotating said furnace and by reclining and inclining saidfurnace; rotating said furnace in order to separate flux and molten; andraising one end of said furnace body in order to pour recovered metal;heating a furnace, in accordance with a control signal obtained from atleast one sensor sensing at least the following: payload temperature;mass of payload; viscosity of said payload; time the payload takes toreach viscosity; atmospheric oxygen content of said furnace; rate ofapplication of energy and cumulative energy applied; identifyingvariables relating to sub-variables; and predicting impact thatvariation of a main variables and a sub-variable has on operation ofsaid furnace.
 35. A method of operating a furnace, comprising: heatingsaid furnace, in accordance with a control signal obtained from at leastone sensor sensing at least the following: payload temperature; mass ofpayload; viscosity of said payload; time the payload takes to reachviscosity; atmospheric oxygen content of said furnace; rate ofapplication of energy and cumulative energy applied; loading saidfurnace with a payload mixture of flux and a material to be melted fromwhich metal is to be recovered; maintaining a controlled furnaceatmosphere, by sealing said furnace with one or more furnace doors;heating said payload mixture until said metal melts; agitating saidmixture so as to promote agglomeration of said metal by rotating andcounter-rotating said furnace and by reclining and inclining saidfurnace; rotating said furnace in order to separate flux and molten; andraising one end of said furnace body in order to pour recovered metal;heating a furnace, in accordance with a control signal obtained from atleast one sensor sensing at least the following: payload temperature;mass of payload; viscosity of said payload; time the payload takes toreach viscosity; atmospheric oxygen content of said furnace; rate ofapplication of energy and cumulative energy applied; and usingalgorithms or look-up tables of variables and sub-variables.
 36. Amethod of operating a furnace, comprising: heating said furnace, inaccordance with a control signal obtained from at least one sensorsensing at least the following: payload temperature; mass of payload;viscosity of said payload; time the payload takes to reach viscosity;atmospheric oxygen content of said furnace; rate of application ofenergy and cumulative energy applied; loading said furnace with apayload mixture of flux and a material to be melted from which metal isto be recovered; maintaining a controlled furnace atmosphere, by sealingsaid furnace with one or more furnace doors; heating said payloadmixture until said metal melts; agitating said mixture so as to promoteagglomeration of said metal by rotating and counter-rotating saidfurnace and by reclining and inclining said furnace; rotating saidfurnace in order to separate flux and molten; and raising one end ofsaid furnace body in order to pour recovered metal; heating a furnace,in accordance with a control signal obtained from at least one sensorsensing at least the following: payload temperature; mass of payload;viscosity of said payload; time the payload takes to reach viscosity;atmospheric oxygen content of said furnace; rate of application ofenergy and cumulative energy applied; obtaining one or more feedbacksignals; making a comparison made between predicted and actualperformance; and deriving a correction signal to effect a change in avariable.
 37. A method of operating a furnace according to claim 36,wherein: a microprocessor is used to monitor and control said operationof said furnace.